I feel like a horrible person right now?

I feel so guilty and horrible right now. I’m 16 and I’m a sophomore in high school. My name is Kyte and don’t or never use this website for anything other then homework help but today, I really need some help. My girlfriend (Jane) and I have been together for 3 years, and before we had been best friends since diapers. Jane parents died 6 months ago in a car crash. And since then she has moved in with her 23 year old sister who is a “poet/teacher“ and “pothead“.

Before I go on you should know that before Jane’s parents died she was this pure Christian girl who was a honors student and that’s why she loved me because I’m a Christian guy, honors student who just wants the best for her. But since her parents have died she has changed. I should also point out that my parents died in a house fire when I was 11 years old so I’m able to relate to Jane and the pain she is going through. I understand.

But over the last 6 months and really the last 4 ? months that she’s been back at school, it has been different. Jane has started smoking pot, being supplied by her pothead sister who has a drug dealing boyfriend.. She doesn’t talk to any of her old friends anymore. except her/my best friend Sid who has turned into a pothead herself. She now hanging around the stoner crowd and she does all these dangerous rebellious things that totally out of her character. And I’ve stayed with her because I love her and understand and she still fun to be around. But I kind of feel like I’m being pushed out her life because I don’t those things like “drugs” that’s she’s doing right now.

So A week ago I went over to her house and it was her, her older sister, her sisters boyfriend and our best friend Sid. When I walked in they were all smoking pot and Jane wanted me to join in knowing I don’t do that type of thing and when I declined and tried to leave she started saying things that got under my skin. Like things you say to a person you know everything about to make them react a certain way. She knew my buttons and she kept pushing them. And the pressure from the people in the room set in and I gave in. I smoked pot for the first time. And I was so “high” I didn’t know what was going on. I was like in a different world. Then Jane gave me a drink that she told me she slip something in to loosen me up, but I was so high I didn’t care and I drank it. The outcome of the night is that we ended up having unprotected sex for the first time. And feel extremely horrible.

I feel like I took advantage of her and that I’m the reason why we broke our promise of celibacy to god. I just feel like if I was a stronger more influential person it would have never happened. And now its all I think about even though I feel horrible that’s all I want to do is relive that night and day and do it over and over again and not change a thing about what happened. All this week I’ve skipped some classes with Jane and although I haven’t smoked pot again, I’ve thought about it and I’m starting not to mind her smoking it around me, which I never let her do before. And we haven’t had sex again but that’s what she wants and that’s what I want. I love Jane so much. Jane and her sister and Sid are like the closest things I have to a real family. And I love them so much

And I just want to know why. Why? Why am I so confused? What can I do about this situation? What should I do?